I think the only other fighter that people could make an argument for would be Royce Gracie and, while it's true that he helped establish the UFC, it was Matt who contributed the most in bringing MMA into the mainstream and getting it taken seriously as a legitimate sport.

If you look at the sport while Matt was on his initial championship run, there was still a lot of pro-wrestling style theatrics that went on outside the octagon. Over-the-top entrances, trash-talking in order to hype a fight, showmanship, etc. At the end of Matt's championship run, the average UFC fighter acted much more respectably and professionally. I think that's very much because of the example Matt set.

Without that, the UFC would have never climbed out of the pit of sideshow novelty.